This invention relates to an improved composition and process for treating substrates to aid in the release of adhesive materials therefrom. Specifically, this invention relates to silicone compositions which cure by the metal catalyzed reaction of SiH groups with olefin groups. More particularly, this invention relates to silicone compositions which cure more rapidly, at lower temperature, and with less residual reactive functionality so that subsequent interaction with applied adhesive is minimized.
The application of curable silicone compositions to substrates such as paper to aid in the release of adhesive materials therefrom is old and well known in the coatings art. For example, the preparation of laminates consisting of a release paper and a sheet material which may be a decorative lamina or a label material coated with a pressure sensitive adhesive is well known. When it is desired to use the coated sheet material and finally affix it, the release paper is stripped off and discarded.
In the preparation of laminates of release paper and a supported adhesive film, one of two processes is normally used. In one process, the off-line process, the silicone composition is coated on paper and cured, then at a subsequent stage, normally after reeling and storing, an adhesive film supported on a sheet material is applied to the coated paper and the assembly is pressed together. In the other process, the in-line process, the silicone composition is applied to paper and cured, the adhesive is then coated on the silicone composition and the supporting sheet material is then applied to the adhesive and the assembly pressed together. While the in-line process is generally more efficient and would normally be more advantageous, it has been found that some adhesives bond or weld to the silicone coated paper if the adhesive is applied to the silicone coated paper within a short time after the silicone composition has been cured. It is believed that residual SiH functionality in the silicone is responsible for the bonding of the silicone and adhesive in the in-line process. The release problem is not usually observed with the off-line process, apparently because the off-line storage period allows for continued reaction and depletion of the reactive functionality of the silicone composition.
It has been suggested by Garden, U.K. Pat. No. 1,518,371, that the in-line weld problem can be reduced by incorporating certain compounds into the adhesive which will inhibit or prevent the bonding interaction between the silicone coating and the adhesive. Although this approach is successful in reducing the weld problem when certain compounds are added to the adhesive, there is still a need for a solution to the problem that does not require the addition of special chemicals to the adhesive component.
Recently there has been interest in applying silicone release coatings to new substrates such as polyethylene sheets which are less stable at the elevated temperatures used to cure the silicone release coating. Consequently there is a need for silicone coatings which cure at reduced temperatures. Of course, lower temperature curing or faster curing silicone coatings are also desired for energy conservation and more efficient production processes.
Silicone compositions which are curable by the metal catalyzed reaction of SiH functional siloxane polymers with vinyl functional siloxane polymers are well known. Kookootsedes et al., U.S. Pat. No. 3,445,420, describes curing compositions using vinyl functional siloxane polymers and generically discloses that any resin or fluid siloxane polymer containing at least one terminally unsaturated monovalent olefin radical per molecule can be used in a wide range of curing compositions useful as elastomers, potting compounds, sealants, dielectric gels, resins, coatings and the like. Kookootsedes et al. further teaches that a methylphenyl-5-hexenylsiloxane-endblocked terpolymer of equal moles of diphenylsiloxane, dimethylsiloxane, and phenylmethylsiloxane units, having a viscosity of 50,000 cs. at 25.degree. C., forms a curable composition when mixed with an appropriate SiH functional siloxane and metal catalyst.
Eckberg, U.S. Pat. Nos. 4,347,346 and 4,476,166, and Melancon, U.S. Pat. No. 4,504,645, further teach vinyl containing siloxane polymers as components of similar curable silicone compositions for release coatings on paper. Again these patents disclose in general terms that any siloxane polymer within a broad genus of olefinorganopolysiloxanes could be used to form similar curable compositions. For example, it is taught that the olefinorganopolysiloxane may include alkenyl radicals such as vinyl, allyl, methallyl, butenyl, pentenyl, and the like.
Oswald et al., U.S. Pat. No. 3,907,852, teach the selective monoaddition of silanes to .alpha.,.omega.-dienes to form .omega.-alkenylsilanes which are then reacted with phosphine to form silylalkyl phosphines.
Hansen et al., U.S. Pat. No. 3,350,351, teach free radical vulcanization of a copolymer consisting of 40 mol percent 3-cyanopropylmethylsiloxane units, 58 mol percent of dimethylsiloxane units and 2 mol percent of 7-octenylmethylsiloxane units.